The present invention relates generally to the measurement of earth formation resistivity from a borehole traversing the formation, and more specifically to the measurement of the resistivity and/or thickness of one or more regions about the borehole.
In many conventional drilling operations, the mud in the borehole is conditioned so that the hydrostatic pressure of the mud column is greater than the fluid pressure of the formations. The effect of this condition is represented in FIG. 1, which shows an exemplary radial distribution of resistivities in a water-bearing bed.
The differential pressure forces mud filtrate into permeable formations traversed by the borehole, whereby the solid particles of the mud are deposited on the borehole wall to form a mudcake of resistivity R.sub.mc. This mudcake usually has very low permeability and considerably reduces the rate of infiltration as it builds. In the portion of the formations immediately about the borehole, however, all of the formation water and some of the hydrocarbons, if present, are flushed away by the filtrate. This zone is called the flushed zone, and its resistivity is expressed as R.sub.XO. Farther out from the borehole the displacement of formation fluids is less and less complete, resulting in a transition zone in which a progressive change in resistivity occurs from R.sub.XO to the resistivity of the uninvaded formation, R.sub.t. The lateral depth of invasion depends in part on formation permeability and is quite variable, ranging from less than one centimeter to several tens of centimeters.
Measurements of R.sub.XO are important for several reasons. When invasion is moderate to deep, knowledge of the R.sub.XO value makes possible more accurate determinations of true resistivity R.sub.t, which is related to hydrocarbon saturation. Also, some methods for computing saturation are entered with the ratio R.sub.XO /R.sub.t. Also, in clean formations, a value of the Formation Factor F may be computed from R.sub.XO and the resistivity of the mud filtrate "R.sub.mf," if the mud filtrate saturation "S.sub.XO " is known or estimated. From F, a value for porosity may be found. A more recent application for R.sub.XO data is in conjunction with a full evaluation of hydrocarbon effects on the Neutron and Density logs, an integral part of the SARABAND.TM. and CORIBAND.TM. services available through Schlumberger Technology Corporation, Houston, Tex. Moreover, the comparison of R.sub.XO and R.sub.t is useful for understanding hydrocarbon moveability.
In addition to recognizing the usefulness of the R.sub.XO measurement, the prior art has been concerned with acquiring information that is indicative of the locations of lateral discontinuities in electrical resistivities of borehole surrounding materials to determine well borehole diameter and depth of fluid invasion. U.S. Pat. No. 2,754,475 (stated inventor: Norelius; issued July 10, 1956), for example, discloses that resistivity measurements may be made at continuously varying lateral depths in the borehole and formation surrounding the borehole by measuring throughout a plurality of borehole locations the average resistivity of formation and borehole fluid contained within respective spherical shells. The shells are established by an electrical field that converges substantially radially through surrounding formations upon an input electrode in the fluid within the well borehole. The borehole radius, depth of invasion, and the location of the uninvaded formation can be determined by identifying any discontinuities which appear in a display.
It has been recognized in the art that a wall-engaging pad device is most appropriate for the measurement of shallow invasion, however. Many such devices have been proposed, one of which is disclosed in U.S. Pat. No. 2,669,688 (Doll, Feb. 16, 1954). One of the embodiments described in the Doll '688 patent includes a three electrode system comprising a current electrode and two potential measuring electrodes, with the current return being on the cable. Measurements made with this system are indicative of the resistivities at different shallow lateral depths of investigation. One depth of investigation is approximately equal to the probable thickness of mudcake on the wall of the borehole, and the other is slightly greater so as to include the mudcake and at least a part of the adjacent portion of the formation that has been invaded by the mud filtrate. Since the presence of mudcake on the wall of a borehole is an indication of invasion of the formation by mud filtrate, proper interpretation of the measurements so made enables permeable formations to be identified. A third measurement at yet another shallow lateral depth of investigation also facilitates the identification of permeable formations. See, e.g., U.S. Pat. No. 2,965,838 (Kister, Dec. 20, 1960).
To accurately measure the value of R.sub.XO, however, the measurement must not be affected by the borehole or must be capable of being corrected. Measurements made at different shallow lateral depths of investigation can be corrected when at least one of the measurements accurately yields mudcake resistivity. See, e.g., the Kister patent. Borehole effects on the R.sub.XO measurements can be minimized, however, by the use of focussing currents to control the path taken by the survey current. Borehole effects, particularly severe as the mudcake thickness increases, arise when the survey current is shunted back to the borehole by the relatively low resistance path formed by the mudcake so that the formation measurement is influenced to a large extent by the mudcake resistivity. Several apparatus have been proposed to overcome this problem. The focussed pad system disclosed in U.S. Pat. No. 2,712,629 (Doll, July 5, 1955) is particularly suitable for use where a minimum to moderate thicknesses of mudcake occurs and in salty muds. In the focussed pad system disclosed in U.S. Pat. No. 3,132,298 (Doll, May 5, 1964), an apparatus is proposed which performs satisfactorily in relatively thick mudcakes. More recently, pad mounted electrode tools have been developed which provide a greater accuracy in obtaining R.sub.XO, especially in thick muds. This new type of well logging tool has been referred to as a spherically focussed apparatus and is described in U.S. Pat. No. 3,760,260 (Schuster, Sept. 18, 1973). The spherically focussed system also has been proposed to determine the lateral thickness of a mudcake as well as R.sub.XO. See, e.g., U.S. Pat. No. 3,973,188 (Attali et al., Aug. 3, 1976). All of the above-mentioned focussed microresistivity tools provide good R.sub.XO measurements under certain conditions, although none provide accurate R.sub.XO values under all conditions.
Wall-engaging pad devices also have found application in the determination of dip. Dip determining devices, known in the art as dipmeters, characteristically employ four pads which are applied against the borehole wall through two perpendicular diameters. Typically, each pad contains a transducer which conducts an investigation of formation characteristics immediately adjacent to the pad. See, e.g., U.S. Pat. No. 3,060,373 (Doll, Oct. 23, 1962). An individual pad may be provided with more than one electrode for the purpose of improving the demarcation between bed boundaries, see, e.g., Doll '373; enhancing dipmeter signals over noise, see, e.g., U.S. Pat. No. 3,521,154 (Maricelli, July 21, 1970); removing variations in speed caused by the so called "yo-yo" effect, see id.; or allowing more detailed correlation of signal features corresponding to vertical changes in formation characteristics, see, e.g., U.S. Pat. No. 4,251,773 (Cailliau et al., Feb. 17, 1981).
Dipmeter tools typically use passively focussed electrodes systems for constraining the surveying current to penetrate laterally for an appreciable distance into the adjacent earth formation. Typically, the focussing current electrode is a metallic surface which forms the major portion of the pad face. Centrally located in the pad face is a recess covered by a layer of insulating material. A survey current electrode is disposed in the recess and is separated from the metal pad proper by the insulating material. Survey current emitted from the survey electrode is caused to penetrate laterally into the adjacent earth formation by the current emitted from the focussing electrode. Additional focussing current may be emitted from the conductive surface of the dipmeter sonde body. Where more than one survey electrode is provided, the additional electrodes also are completely surrounded by the focussing electrode such that the respective survey current beams are focussed as described above by current emitting from the focussing electrode. The current return may be via a return electrode B located on the lower end of the multi-conductor cable as in the Doll '373 patent, or via a tool body member as in the Cailliau et al., patent.
In the prior art dipmeter systems, the survey current beams are focussed to penetrate relatively deeply into the earth formation in front of the pad member so that an appreciable portion of the electrical resistance experienced by the beam in the earth formations will be contributed by the uncontaminated portion of the formations, even though this zone is spaced from the borehole proper by a mudcake and an invaded zone. Moreover, the focussing and depth of penetration of all survey currents typically are essentially identical to facilitate dip determination and, in the case of a multiple electrode pad tool, to facilitate noise cancellation, speed correction, and/or more detailed correlation of features.